Field of the Invention
The present invention relates, in general, to borehole electromagnetic exploration or tomography (EM tomography) and more particularly, to an induction type broadband 3-component borehole magnetic measuring sensor able to accurately and precisely measure a broadband magnetic field about x, y and z axes using a three-dimensional (3D) model within a borehole by monitoring natural variations in the earth's magnetic field within the borehole or based on EM tomography using the borehole, such that the measuring sensor is applicable to energy resource fields such as petroleum and coal, mineral resources fields, civil engineering, and environmental fields.
The present invention also relates to a borehole EM tomography method using the induction type broadband 3-component borehole magnetic measuring sensor able to accurately and precisely measure a broadband magnetic field about x, y and z axes using a 3D model within a borehole, such that the borehole EM tomography method is applicable to energy resource fields such as petroleum and coal, mineral resources fields, civil engineering, and environmental fields.
Description of the Related Art
In the related art, proposed are borehole electromagnetic (EM) tomography methods of examining the geological features or environment of the underground by disposing an electromagnetic sensor within a borehole. These EM tomography methods are used to monitor variations in the earth's magnetic field or explore for energy resources such as petroleum or coal or mineral sources such as metals, and are applicable to civil engineering and environmental fields.
Reference will be made to FIG. 1 schematically illustrating the overall configuration of a borehole EM tomography system 10 of the related art.
As illustrated in FIG. 1, the borehole EM tomography system 10 includes a transmitter 13 deployed in one borehole 11 and a receiver 14 deployed in the other borehole 12 formed separately from one borehole 11. The receiver 14 receives signals from the transmitter 13. A surface unit 15 deployed on the surface receives and analyzes signals received from the transmitter 13 and the receivers 14, thereby exploring a corresponding area for geological features, main resources buried therein, or the underground environment.
For the exploration, the transmitter 13 can be positioned not only within the borehole but also on the surface, and the receiver 14 can be positioned not only within the borehole 12 but also within the borehole 11 in which the transmitter 13 is deployed.
Specifically, as an example of the related art of the above-mentioned borehole EM tomography system, U.S. Pat. No. 7,030,617 (Apr. 18, 2006) proposed “SYSTEM, APPARATUS, AND METHOD FOR CONDUCTING ELECTROMAGNETIC INDUCTION SURVEYS.”
Reference will be made to FIG. 2 schematically illustrating the overall configuration of the electromagnetic induction survey system proposed in U.S. Pat. No. 7,030,617.
More specifically, as illustrated in FIG. 2, in the electromagnetic induction survey system 20 proposed in U.S. Pat. No. 7,030,617, a transmitter 22 for generating magnetic moment is deployed within a borehole 21, a receiver 23 for receiving a magnetic field generated from the transmitter 22 is deployed within another borehole 24 adjacent to the borehole 21. An auxiliary receiver 25 is deployed adjacent to the transmitter 22 to detect the magnetic field generated from the transmitter 22 and attenuation caused by a conductive casing. A surface unit 26 receives and analyzes signals transmitted from the transmitter 22, the receiver 23 and the auxiliary receiver 25, thereby exploring a corresponding area for geological features, main resources buried therein, or underground environment.
Therefore, according to the electromagnetic induction survey system 20 proposed in U.S. Pat. No. 7,030,617, it is possible to determine moment caused by the conductive casing based on the ratio between the magnetic field measured by the auxiliary receiver 25 and the magnetic moment generated by the transmitter 22, thereby obtaining a more accurate measurement.
In general, the geological features have a very sophisticated 3-dimensional (3D) structure. It is required to measure and analyze the direction of a magnetic field in three directions, i.e. x, y and z axis directions, using a 3D model in order to accurately examine the environment of an underground space. However, U.S. Pat. No. 7,030,617 as mentioned above merely proposed the configuration of the transmitter that generates magnetic moment and the receiver that receives the magnetic moment, but did not disclose or suggest any of the above-mentioned technologies for measuring and analyzing three components of a magnetic field.
In addition, other examples of the related art about the above-mentioned borehole EM tomography system include “DEPTH ESTIMATION APPARATUS FOR STEEL EMBEDDED DEEP FOUNDATION USING DIGITAL 3-COMPONENT FLUX GATE MAGNETIC MEASURING DEVICE” proposed in Korean Patent No. 10-0563542 (Mar. 16, 2006) and “APPARATUS AND METHOD FOR DETECTING REINFORCING ROD IN CONCRETE FOUNDATION PILE USING 3-axis MAGNETIC FIELD MEASUREMENT WITHIN BOREHOLE” proposed in Korean Patent No. 10-0264630 (Jun. 2, 2000).
More specifically, first, Korean Patent No. 10-0563542 disclosed a depth estimation apparatus for a steel embedded deep foundation using a digital 3-component flux gate magnetic measuring sensor. In this depth estimation apparatus, the flux gate sensor is inserted into a borehole to detect an induced magnetic field generated from an iron rod and subsequently outputs a digital signal. The depth estimation apparatus also includes a means for converting information about the digital signal generated by the flux gate sensor to RS-422, RS-232 and USB communication protocols. The information detected by the flux gate sensor can be thereby stored and output using a portable computer such as a notebook computer. Accordingly, it is possible to detect and transmit a greater amount of data using the digital flux gate sensor.
In addition, Korean Patent No. 10-0264630 disclosed an apparatus and method for detecting a reinforcing rod in a concrete foundation pile using a 3-axis magnetic field measurement within a borehole. According to this document, a 3-axis magnetic sensor is inserted into a borehole and is moved upwards and downwards to measure an induced magnetic field of an iron rod within a foundation pile, and simultaneously, the depth of the 3-axis magnetic sensor is measured. The depth of the bottom of the foundation pile is measured by storing the measured value of the induced magnetic field of the iron rod and the measured depth of the 3-axis magnetic sensor in a computer.
As described above, Korean Patent Nos. 10-0563542 and 10-0264630 disclosed the technologies for performing a 3D measurement by inserting the 3-axis magnetic sensor into the borehole. However, these technologies proposed in Korean Patent Nos. 10-0563542 and 10-0264630 are intended to measure the bottom depth of the iron rod embedded in the concrete foundation pile. Since it is assumed that the borehole is close to the foundation pile, a specific high frequency band may be used and a wide frequency band is not necessarily required. Therefore, it is possible to use a commercially available flux gate type 3-axis magnetic sensor.
When EM tomography is performed using an electromagnetic wave having a specific frequency in a specific ground environment, a penetration or skin depth δ indicating an possible depth of investigation can be expressed by following Equation 1:
                              δ          =                      503            ⁢                                          ρ                f                                      ⁢                          (              m              )                                      ,                            Formula        ⁢                                  ⁢        1            
where ρ indicates the electric resistivity (ohm-m) of a geological stratum, and f indicates a frequency (Hz).
More specifically, for example, in the case that EM tomography is performed at an area where the electric resistivity of the geological stratum is 100 ohm-m, the skin depth δ is about 500 m when a 100 Hz frequency is used. When a 10,000 Hz frequency is used, the skin depth δ is about 50 m.
In Korean Patent Nos. 10-0563542 and 10-0264630, the borehole is generally positioned as close as possible to the foundation pile such that the distance between the borehole and the foundation pile is several meters. An electromagnetic wave in a high frequency band ranging from tens to hundreds of kilohertz is available. Consequently, in this case, the purpose of the exploration can be realized using a flux gate magnetic sensor.
In contrast, the borehole EM tomography must be carried out in a variety of geological environments, in which a variety of distances ranging from tens to hundreds of meters is applied between the two boreholes, and the electrical resistivity of the geological stratum ranges from hundreds to tens of thousands of ohm-m. Thus, the frequency of an electromagnetic wave used in such environments must be measurable in a broadband ranging from several Hz to several hundreds of kHz with a very high level of sensitivity. In this case, the above-mentioned flux gate type EM tomography receiver can hardly satisfy both broadband and sensitivity requirements.
Furthermore, like U.S. Pat. No. 7,030,617, neither Korean Patent No. 10-0563542 nor Korean Patent No. 10-0264630 disclosed any specific configuration of the magnetic sensor able to obtain a 3D measurement.
In the borehole EM tomography system for exploring an underground environment for geological features or resources buried therein, the magnetic sensor must include several auxiliary devices in addition to the 3-axis magnetic sensor for detecting three components, i.e. x, y and z axis components, of a magnetic field. The auxiliary devices may include a means for controlling the magnetic sensor, a means for locating the magnetic sensor, a communications means for transmitting a signal received from the magnetic sensor, and a means for supplying electric power to the magnetic sensor for its operation. However, the above-mentioned related art proposed no specific configurations for the magnetic sensor.
Therefore, in order to overcome the above-described problem of the related art, it is required to provide a novel borehole magnetic measuring sensor applicable to a borehole EM tomography system for exploring the underground environment for the geological features, main resources buried therein, or the like and a borehole EM tomography method using the same measuring sensor. It is preferable that the measuring sensor is fabricated as an integrated module for the ease of positioning within a borehole, and can accurately and precisely measure a magnetic field having a broadband frequency within the borehole about x, y and z axes using a 3D model. However, neither an apparatus nor a method satisfying all of these requirements has been introduced yet.
The information disclosed in the Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or as any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.